<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let arr = [ 1,2,3,9,8,7,6,5,4,3,3,2,1]
    // let newArr = []
    // for (let i = 0; i < arr.length; i++) {
    //   if(newArr.indexOf(arr[i]) == -1){
    //     newArr.push(arr[i])
    //   }  
    // }

    
    let newArr = Array.from(new Set(arr))
    console.log(newArr);
  </script>
</body>
</html>